This invention relates to cleaning immersed ultrafiltration or microfiltration membranes with a chemical cleaner.
Membranes are used for separating a permeate lean in solids from tank water rich in solids. Typically, filtered permeate passes through the walls of the membranes under the influence of a transmembrane pressure differential between a retentate side of the membranes and a permeate side of the membranes. Solids in the feed water are rejected by the membranes and remain on the retentate side of the membranes. The solids may be present in the feed water in solution, in suspension or as precipitates and may further include a variety of substances, some not actually solid, including colloids, microorganisms, exopolymeric substances excreted by microorganisms, suspended solids, and poorly dissolved organic or inorganic compounds such as salts, emulsions, proteins, humic acids, and others.
Over time, the solids foul the membranes which decreases their permeability. Any solid can contribute to fouling and reduced membrane permeability, and the fouling may occur in different ways. Fouling can also occur at the membrane surface or inside of the pores of the membrane. To counter the different types of fouling, many different types of cleaning regimens may be used. Such cleaning usually includes both physical cleaning and chemical cleaning.
For physical cleaning, permeation through the membranes is typically stopped momentarily. Air or water are flowed through the membranes under pressure to backwash the membranes. The force of the backwash physically pushes solids off of the membranes. Typically, the membranes are simultaneously agitated, for example by aerating the feed water around the membranes with large, scouring bubbles to assist in shearing solids from the surface of the membranes. Such back washing and agitation is partially effective in removing solids from the surface of the membranes, but is not very effective for removing solids deposited inside the membrane pores and is almost ineffective for removing any type of solid chemically or biologically attached to the membranes.
Accordingly, fouling continues despite regular physical cleaning. This continued fouling is countered by cleaning with a chemical cleaner. For example, the membranes may be soaked in one or more cleaning solutions either in the process tank (after it has been drained and filled with chemical cleaner) or in a special cleaning tank. These methods, however, require either large volumes of chemical cleaner (to fill the process tank) or the expense of providing special cleaning tanks and means to move the membranes to the cleaning tank. These methods also disrupt permeation for extended periods of time.
Other methods involve backwashing the membranes with a chemical cleaner. Examples of such methods are described in U.S. Pat. No. 5,403,479 and Japanese Patent Application No. 2-248,836 in which chemical cleaning is performed without draining the tank or removing the membranes from the tank. Permeation is stopped and the membranes are cleaned by flowing a chemical cleaner in a reverse direction through the membranes while the membranes are simultaneously agitated. Although effective, these methods leave residual chemicals in the tank. In wastewater applications, the chemicals interfere with useful biological process in the tank water. In drinking water applications, the chemicals pass through the membranes when permeation is resumed resulting in unwanted concentrations of chemicals in the permeate. Further, some chemical cleaner disperses in the tank water during the cleaning event thus increasing the amount of chemical cleaner required.
French Patent No. 2,741,280 describes another method of backwashing membranes with a chemical cleaner. In this method, the tank water is drained before the chemical backwash begins. When the chemical backwash is over, the cleaner is drained from the tank and the tank is refilled. In this way, the chemical cleaner does not contaminate the tank water or permeate. In a typical municipal installation, however, the tank may range from 1 m to 10 m in depth. The chemical cleaner inside the lower membranes or the lower portions of vertical membranes may be subject to a local pressure up to 100 kPa higher than the local pressure of the chemical cleaner inside the upper membranes or the upper portions of vertical membranes. Since the flow of chemical cleaner through the membranes is dependant on the local pressure of the chemical cleaner inside the membranes, the flow rate of chemical cleaner varies considerably between the upper and lower membranes. As a result, either insufficient cleaner is supplied to the upper portions of the membranes or excess cleaner is supplied to the lower portions of the membranes.
It is an object of the present invention to provide a method and apparatus for chemical cleaning of immersed microfiltration and ultrafiltration membranes.
According to an embodiment of the invention, the tank is first drained, a chemical cleaner is backwashed through the membranes, the cleaner is preferably removed from the tank, and the tank is refilled so that permeation may continue. A backwash pump which drives the chemical cleaner is controlled by a speed controller which is in turn connected to a programmable logic control and, preferably, pressure and flow indicators. The backwash pump is operated to supply the chemical cleaner to the membranes in pulses.
The pressure of the pulses is selected to be high enough to reduce the relative size of the local pressure differentials in the system, including local pressure differentials between upper and lower membranes or portions of membranes. The duration and frequency of the pulses is chosen to provide an appropriate contact time of the chemical cleaner, preferably without allowing the membranes to dry between pulses and without using excessive amounts of chemical cleaner.